How many calories do I burn by walking?

The calories you burn are equivalent to how much energy you use while walking, which is mainly a factor of your weight and the distance that you walk. A simple rule of thumb is 100 calories per mile for a 160 pound person. You burn more calories per mile at very low speeds because your momentum isn't helping to carry you along. At very high speeds, you use more muscle groups that burn extra calories with each step.

If you want to burn calories for weight loss, remember that the weight you gain or lose is mostly determined by a balance of the calories you eat versus the calories you burn each day. To lose weight, you need to burn more and/or eat fewer calories. A pound of fat equals 3500 calories, so to lose 1 pound in a week, you need to burn 3500 (500 per day) more calories than you eat.

There are many online calculators to help you determine how many calories you burn doing different physical activities:
Walking Calories Calculator
Pedometer Steps Calorie and Distance Calculator
Calories per Day Calculator